This invention relates generally to magnetic resonance imaging (MRI) and, more particularly, the invention relates to multiple inversion recovery imaging.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Multiple inversion recovery (MIR) for selectively imaging blood grew out of the SIR (Selective Inversion Recovery) technique introduced by Nishimura et al., MRM, 4, 193-202 (1987). In SIR the blood upstream of the imaging region is inverted, and then after an inflow time a projection is made of this imaging region, into which the inverted blood has flowed. The positioning is made of this imaging region, into which the inverted blood has flowed. The positioning of these pulses is displayed in FIG. 3A. A projection is subsequently made of the imaging region without inverting the inflowing blood, and the projections are subtracted. The sequence thus produces an image of the blood that has flown into a large slab during some inflow time.
SIR has several advantages over competing time-of-flight techniques, notably that there is no nonlinear MIP projection imposed upon the data, which can cause mistaken estimation of stenosis. In addition, tortuous vessels are accurately imaged, and the sequence is easily gated, avoiding pulsatile flow artifacts which are also accentuated around stenosis.
However, there are several problems with SIR, each frame of data requires two excitations, and the subtraction can cause misregistration artifacts due to the large static signal. To reduce this large static signal, Dixon et al., MRM, 18, 257-268 (1991), proposed doing multiple inversions before acquisition, and also proposed doing the imaging in a non-subtractive fashion if the suppression was adequate. Dixon et al. demonstrated a non-subtractive image of the carotids using 2 inversions. Non-subtractive inflow imaging avoids misregistration problems, and allows image formation in half the excitations of SIR. Dixon et al. first saturated a slab and then performed 2 inversions, which should null (i.e., substantially reduce the longitudinal magnetization) muscle and fat after an inflow time. A projection was then taken of the slab, which should only display the blood that has flowed in to the slab since the saturation. However, imperfections in the multiple inversion technique due to various inhomogeneities can cause inaccurate background nulling. As blood is often only 1/20 of the tissue signal, even allowing 5% of the signal background to remain creates a signal-to-noise ratio of 1:1 when projecting through a slab.
The present invention remedies the difficulties in performing non-subtractive angiograms, providing robust static tissue suppression in human patients on the order of 60 dB. This allows consistent and easy imaging of vessels and their stenoses in various regions of strong flow, such as the carotid and renal arteries.